Volume rendering is a general method to composite three-dimensional (3D) digital volumetric data onto a two-dimensional (2D) image. The quality and appearance of the resulting image can vary widely from one volume rendering engine to another due to the choice of different tradeoffs in different implementations.
For volume rendering, structures may be masked from the volume data. The mask may be drawn by a user of the rendering system or a predetermined structure.
Examples of masks include a punch mask and a show mask. A punch mask has a transfer function with an opacity of zero everywhere assigned to the mask and masks with a different transfer function (i.e., not zero everywhere) may be referred to as show masks.
Punch masks are used in volume rendering to avoid rendering masked structures. Geometric punching information is converted to a punching volume. The punching volume defines, for each voxel, whether the voxel is masked out.
Thus, when performing segmented volume rendering, two kinds of segmentation boundaries occur. One is a boundary of a segmentation mask and the other is a boundary surface that is implicitly defined by a transfer function which separates regions that have zero and nonzero opacity.